PROJECT SUMMARY/ABSTRACT Ovarian cancer is the deadliest gynecologic malignancy. The long-term goal of this research is to develop new strategies for treating chemoresistant ovarian cancer. The objective of this proposal is to investigate the mechanisms by which histone deacetylase inhibitors (HDACi) and poly ADP ribose polymerase inhibitors (PARPi) are synergistic in CCNE1 (cyclin E) amplified non-BRCA mutated tumors. Cyclin E amplified ovarian tumors have enormous replicative stress and genomic instability and depend on robust DNA repair mechanisms, including upregulation of BRCA1, for survival. Further, cyclin E amplified ovarian cancers are relatively resistant to DNA damaging drugs and have poor clinical outcomes. PARPi have shown clear advantage in BRCA mutated ovarian cancer. PARPi are far less effective in non-BRCA phenotypes, but could be improved in rational combinations with other drugs. Our group has generated multiple lines of evidence to support HDACi-based drug combinations for the treatment of chemoresistant ovarian cancer. We now have preliminary data to show that the combination of the HDACi panobinostat and the PARPi olaparib is synergistic in vitro and in vivo, and induces more downregulation of E2F1 and E2F1 targets (cyclin E and BRCA1) than each drug alone in cyclin E amplified ovarian cancer cells. This unexpected finding led us to ask how HDACi and PARPi drugs synergize at the transcriptional level. Emerging evidence reveals that HDACi work through disruption of BRD4 binding to promoters and enhancers involved in transcriptional elongation of gene targets. Recent exciting results by another group suggest a novel role of PARPi in targeting transcriptional elongation. We now have an unprecedented opportunity to use multiple state of the art genomic methods, unique ovarian cancer cell lines, and clinically pertinent patient-derived xenograft (PDX) models to purse the following hypothesis and aims. Our central hypothesis is that both HDACi and PARPi inhibit transcriptional elongation and pause key genes involved in oncogenic function and thus, induce synergistic effects in vulnerable tumors. Aim 1 will define the mechanisms by which HDACi and PARPi synergistically regulate transcription in cyclin E amplified ovarian cancer. Aim 2 will investigate the therapeutic potential of a regimen of HDACi and PARPi drugs. The proposed work will explain how the HDACi and PARPi combination induces a BRCA-like phenotypic switch and why the regimen of panobinostat and olaparib is a rational strategy for treating cyclin E amplified non-BRCA mutated ovarian cancer. Our approach will lead to the discovery of new mechanistic insights into the synergistic role of HDACi and PARPi on transcriptional profiles and will generate essential preclinical data for a Phase I/II clinical trial of panobinostat and olaparib in cyclin E amplified and other non- BRCA mutated chemoresistant ovarian tumors. The long-term potential impact on clinical outcomes is high for extending the benefits of PARPi to most women diagnosed with ovarian cancer and for improving clinical outcomes for this deadly disease.